In a relational database, information is stored in indexed tables. A user retrieves information from the tables by entering input that is converted to queries by a database application. The database application submits the queries to a database server. In response to a query, the database server accesses the tables specified in the query to determine which information within the tables satisfies the queries. The information that satisfies the queries is then retrieved by the database server and transmitted to the database application and ultimately to the user. Queries may also be internally generated and executed by a database system for performing administrative operations.
For any given database application, the queries must conform to the rules of a particular query language. Most query languages provide users with a variety of ways to specify information to be retrieved. For example, in the Structured Query Language (SQL), the following query requests the retrieval of the information contained in all columns of rows in table T1 in which the value of column a is 2: